Yo Stray, thanks for the guide/list. How can you have both 16 orbs and katanas? Like, what would the button input be? This is one of the more confusing shmups on account of the fact that the button config is so limited.
-Thanks in advance

EDIT: Nevermind I got it.

You can't the achievement description is wrong, here's what I wrote on true achievements it does take some practice, getting 16 katanas or orbs on there own will not unlock it. Couldn't unlock this till a Japanese friend noticed the error and helped me out.

"First off I'll start by saying that the achievement description is wrong for the English version it should read:

"Equip a total of 8 katanas and 8 steel orbs on your Phantom."

Anyone who views the achievement description for the Japanese version will notice the mistake. Also it doesn't take much to notice you can only have 16 items around your ship at any moment.

Once you know this it's rather easy to do, first off however this achievement can only be done in "Slash" mode and it's slightly easier with only one life as you can have more phantom mode energy when on one life.

Following guide is for the DEFAULT CONTROLS

First off you need to get eight steel orbs and turn them into katanas, to do this in the ship form shoot with the “RT” shot and kill the enemies to collect steel. You want to collect enough steel to make exactly eight steels orbs, you can see how much steel you have collected by the small circles that appear in the ring around your ship. The eighth circle is the one you want to stop at, just before you complete a full half circle around your ship.

(Note: for the next part the more phantom energy you have the longer you can stay in phantom form making this achievement easier to do. Also do not change back to the ship form else you will lose any items you have around your ship.)

Now once you have eight circles around your ship press the “X button” to swap to phantom form this will convert the collected steel to steel orbs. Now comes the tricky part you need to launch all eight steel orbs by press and holding the “A button” into an enemy to turn them into a katanas, each one must hit an enemy to change into a katana. If one of the orbs fails to change into a katana you will have to start over.

You will likely now be low on phantom form energy, but while still in phantom form and with eight katanas now flying around you. You now need to use the “RT” shot of the phantom form to collect steel again this time to get eight more steel orbs to appear around you. Unlike in ship form however in phantom form as soon as you get enough steel for a steel orb it will automatically appear around you.

As soon as you get eight steels orbs and eight katanas around you the achievement should pop, but be sure not to fire or lose any of the katanas as you collect the final set of steel orbs, else you’ll be starting over again, same if you don’t have eight steel orbs before your phantom form energy runs out.

This will take a number of attempts I lost count of how many times I had too many or too few katanas, I also couldn’t do this unless I had only one life as I just didn’t have enough phantom form energy to collect the last few steel orbs otherwise.

One last tip for collecting steel the closer you are to an enemy when you get them the more steel you get from them.

Oh and items floating around your ship and phantom disappear over time, so speed really is required here.

I got this achievement on the first level after killing myself twice at the very start to increase my phantom form energy bar."

This guide needs to be alot more detailed in places, as it just doesn't provide much information. Big Catch is a real pain to unlock and I can't find a good guide for it on the net.